In order to recover oil and gas from fields located below seabed level, a drilling operation is first undertaken to produce a well. Once the well has been drilled, it is conventional for installation equipment such as valves, for example, to be positioned over, and connected to, the well in order to control the recovery of the oil or gas to the surface. With this type of installation the equipment sits exposed on the seabed, and it is possible for the equipment to be damaged by dropped objects, anchors, trawler nets and other similar hazards. It is therefore advantageous to provide some type of protection for such subsea installations.
Attempts at protecting the installations have included steel framed, fixed geometry structures, which are lowered over the installation by a crane from a supply vessel on the surface. Due to their size and fixed geometry, these structures occupy a significant amount of space on the supply vessel, which normally means that the vessel may only carry one structure at a time. The deployment of these structures can also be restricted in bad weather conditions as a result of the large hydrodynamic forces created by their size and also by their fixed geometry.
In order to attempt to address the problems of such fixed geometry structures, collapsible structures have also been utilised, and such structure is shown in International Patent Publication No WO03/071092. As they are collapsible, the structures can be lowered to the installation site in their collapsed states, before being set up over the installation on the seabed. Although an improvement over fixed geometry structures, these collapsible structures include components such as folding legs and anchors which still restrict the storage possibilities on vessels and also create significant hydrodynamic forces when being lowered to the site. Furthermore, these known fixed and collapsible structures are provided with openings—usually between the legs of the structure—to allow access to the installation equipment. With such openings, there still exists the possibility that one of the subsea hazards previously listed could enter these openings and interfere with and/or damage the installation equipment. This multiplicity of openings can be closed by protective plates or grills, but this adds to the complexity of the design and/or further exacerbates the hydrodynamic loadings during launch.